eruption episodes and magma recharge events in andesitic systems: mt taranaki, new zealand

Journal of Volcanology and Geothermal Research 177 (2008) 1063–1076

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Journal of Volcanology and Geothermal Research

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Eruption episodes and magma recharge events in andesitic systems: Mt Taranaki,New Zealand

Michael B. Turner a,⁎, Shane J. Cronin a, Ian E. Smith b, Robert B. Stewart a, Vince E. Neall a

a Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North, New Zealandb School of Geography, Geology and environmental Sciences, University of Auckland, PB92019, Auckland, New Zealand

⁎ Corresponding author. Tel.: +64 6 356 9099x4821; fE-mail address: [email protected] (M.B. Turn

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.08.001

a b s t r a c t
a r t i c l e i n f o
Article history:
Eruption episodes, where a Received 22 July 2008Accepted 4 August 2008Available online 11 August 2008
Keywords:magma rechargecompositional zoningtitanomagnetiteplagioclaseclinopyroxeneamphiboleMt Taranaki

series of eruption events are generically related, can include the eruption of awide spectrum of volcanic activity over decadal periods. This paper concentrates on the opening phases of aneruption episode which occurred approximately 1800 yrs BP from Mt Taranaki, New Zealand. These eventsspanned the eruption of differing bulk compositions and styles from two distinct vent locations; an andesiticsub-plinian eruption from the summit vent and a scoria cone-building eruption of basaltic magma from asatellite vent. Compositional profiles and zoning textures of plagioclase, amphibole and clinopyroxenephenocrysts from the opening andesitic event show evidence of magma mixing and subsequentcrystallisation just prior to the initiation of the eruption episode. Titanomagnetite grain morphology andTi variation suggest that the magma mixing event occurred within a few days to weeks before the eruptionacting as a trigger for it. We present a magmatic model which is constrained by the petrological observationsand eruptions of the episode. In this model magma differentiation at depth causes its rise and recharging of amid-crustal magma storage area at 5–7 km. Although the recharging magma differed slightly in oxygenfugacity and temperature, it was compositionally and physically similar enough to the residing andesiticmagma to allow efficient mixing. The petrological characteristics described here can be readily observed andenable identification of mixing events in other recent eruption episodes.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Studies that provide a holistic model of the magmatic system thatfeeds volcanic eruptions are becoming more important for definingand assessing volcanic hazards. Magmatic models of volcanic systems,especially within arc volcanic settings are commonly based on lavaflow sequences, because lava samples are less likely to be affected bypost-depositional alteration (e.g., Price et al., 1999). Much less suitablefrom a petrologic perspective, but often chronologically betterconstrained, are pyroclastic deposits, especially in intermediate todistal sequences.

Studies of the pyroclastic record at many volcanoes are focussed onthe deposits of larger (Nsub-plinian) eruptions (i.e., Crandell andMullineaux, 1975; Alloway et al., 1995; Donoghue and Neall, 1996).This is often because these are most easily identifiable, coarser andthus least-altered and more readily sampled, in comparison to themore numerous minor units that require detailed fine-scale studies.Thus petrological data of volcanic processes (magmatic and eruptive)at any given centre are commonly limited to only one end of thepossible eruption magnitude/frequency spectrum. Integration of

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petrological studies of all eruptives throughout the eruption historyof a volcano is essential for a complete understanding of volcanicsystems and also for accurate forecasting of eruption progression.

1.1. Mt Taranaki

The near-symmetrical stratocone of Mt Taranaki/Egmont (2518 m)dominates the landscape of the western North Island of New Zealand(Fig. 1). It is located in a “back-arc” position, approximately 200 kmwest of the Hikurangi Trough, in-turn located at the southernmost endof the Tonga–Kermadec subduction system. Eruptives from MtTaranaki are relatively potassium-rich compared to the andesiticvolcanoes of the Taupo Volcanic Zone, 140 km to the east (Neall et al.,1986; Price et al., 1992; Stewart et al., 1996; Price et al., 1999), hencethe magmas are classified as high-K andesites in the Gill (1981)system. Mt Taranaki has been erupting for at least 125000 yrs. Ahighly detailed eruption chronology for the Holocene has beenestablished through studies of tephra-fall deposits in downwind soilenvironments, with correlations to more complete near-ventsequences (Druce, 1966; Neall, 1979; Alloway et al., 1995).

In this study, we have extracted a record from a sequence of“minor” eruption products (0.15–0.01 km3; total volume) of MtTaranaki to establish a model for the patterns of small-scale activityat a typical andesitic stratovolcano. Here we concentrate on a

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Fig. 1. Orthophotograph showing Mt Taranaki and the distribution of the Curtis Ridge unit 1 eruptive. 50, 100 and 150 mm isopachs are shown. Spot thicknesses in mm. Localitiesmentioned in the text are labelled.

1064 M.B. Turner et al. / Journal of Volcanology and Geothermal Research 177 (2008) 1063–1076

sequence of pyroclastic deposits erupted ~1800 yrs BP, informallytermed the Curtis Ridge eruption episode. Our aim is to develop amodel of themagmatic processes and possible eruption-drivers of thisand similar volcanoes.

2. The Curtis Ridge eruption episode

One of the more complete sequences of Holocene tephra-falldeposits on Mt Taranaki occurs on its eastern flanks c. 3.5 km from thesummit (Fig. 1: Pembroke Road, 39°18′35″South 174°06′04″East). Thisis a type locality for a number of sub-plinian pyroclastic fall units fromMt Taranaki (Neall et al., 1986; Alloway et al., 1995). Within thisoutcrop is also a group of visually unremarkable deposits, less than10 cm in total thickness, which had not previously been mapped,named or correlated to other localities. The units are here described asthe Curtis Ridge eruption episode (CR) named after the type locality(The Curtis Ridge; Fig. 1) where the stratigraphy of all units within thiseruption episode is represented. The CR units are underlain by acharcoal-bearing pyroclastic flow deposit dated at 1950±90 Cal yrs BP(NZ3886C, Neall and Alloway, 1986). This deposit is overlain byweathered, fine-medial ash and a small, unrelated pumiceous depositin which a paleosol is weakly developed. Based on estimates of soildevelopment within theMt Taranaki environment (c.f., Lees and Neall,1993) we interpret the age of the eruption episode to be c. 1800 Cal yrsBP. Detailed field mapping extending out from the Pembroke Roadlocality was used to determine the aerial extent of CR deposits, thus

establishing a full stratigraphy for this episode and providing the basisfor petrologic sampling.

2.1. Stratigraphy of the Curtis Ridge events

The fall deposit of CR unit 1 (Fig. 2), comprises approximately 60%dense grey andesite lapilli, together with 40% greyish-brown pumicelapilli (approx. 1.5–1.9 g cm−3). The dense grey clasts showcharacteristic radially fractured and/or bread-crusted features,which suggest that they were a juvenile component of this eruption.They are also of very similar bulk composition to associated pumiceclasts (Table 1). We therefore interpret these clasts to be juvenile andderived from degassed magma within the upper conduit, or as a lavadome situated over the vent. These dense juvenile clasts weredisrupted and incorporated in the eruption column during ejectionof more gas-rich (pumice-producing) magma of the lower conduit,similar to the model of Platz et al. (2007) for the most recent pumice-producing eruption from this volcano. The proportion of pumicegradually increases upward to 70% through the unit. The pumice hasgreater density (approx. 1.2–1.5 g cm−3) than typical lapilli from thelarger sub-plinian tephras at Mt Taranaki (approx. 0.8–1.1 g cm−3). Thedull brown, vesiculated, glass-dominated groundmass containsmicrolites of plagioclase, clinopyroxene, titanomagnetite±amphibole(Fig. 3a). The phenocryst assemblage (approx. 45 vol.%) comprisesplagioclase, clinopryoxene, amphibole and titanomagnetite. This issimilar to the dense juvenile clasts within the deposit, with the

Fig. 2. Partial composite stratigraphic record of Holocene eruptions, eastern MtTaranaki. Modified from Neall and Alloway (1986).

Table 1Selection of whole-rock major and trace element data for Taranaki eruptives. Majorelement concentration: wt.% at 110 °C. Stratigraphic groups: Lava flows from easternside of the current crater estimated to be b1500 yrs BP (Price et al., 1999); CR=CurtisRidge units. Clast type: BA: Basaltic-andesite, A: Dense rock andesite, P: Pumiceousandesite

Sample EB-4b E03-61b EB-24c E03-19 EB-4p E03-50 E03-57 EB-26

Rock type BA BA A A P A A A

Stratigraphicgroup

CRunit 2

Scoriacone

CRunit 1

CR unit 1 CRunit 1

b1.5 kaBP lavaflow

b1.5 kaBP lavaflow

b1 kaBP lavaflow

Major elements (wt.%)SiO2 51.33 51.13 54.59 54.77 53.60 54.72 54.29 55.59TiO2 0.99 1.05 0.82 0.82 0.86 0.90 0.92 0.87Al2O3 17.78 17.98 17.30 17.10 17.44 17.76 17.77 17.32FeOtotal 9.47 9.85 8.13 8.05 8.16 8.15 8.39 7.51MnO 0.17 0.18 0.17 0.17 0.17 0.16 0.16 0.16MgO 4.45 4.39 3.80 3.85 3.86 3.38 3.45 3.27CaO 9.69 9.87 8.38 8.53 8.77 8.04 8.19 7.60Na2O 3.42 3.42 3.31 3.66 3.15 3.27 3.26 3.79K2O 1.87 1.86 2.17 2.20 2.08 2.34 2.31 2.70P2O5 0.28 0.28 0.25 0.25 0.27 0.27 0.27 0.30LOI 0.23 −0.05 0.05 0.25 0.55 0.05 0.14 0.26Total 99.89 99.9 99 99.9 99.46 99.07 99.26 99.63

Trace elements (ppm)Ba 726 740 783 756 785 836 836 798Rb 40 43 51 47 48 55 55 59Sr 608 628 598 604 604 599 603 625Pb 5 9 24 13 14 20 18 15La 13 10 11 11 11 16 13Ce 23 15 69 45 52 65 61 32Y 21 19 21 19 21 21 23 21Zr 92 104 101 104 103 111 106 127Nb 6 7 5 7 6 6 5 7Sc 25 21 22 21 24 20 18 17V 289 227 220 227 226 230 246 207Cr 53 46 58 46 52 8 7 7Ni 18 19 15 19 15 6 8 12Cu 104 29 30 29 92 97 106 74Zn 77 85 101 85 82 81 80 72Ga 20 19 19 19 20 20 19 20

1065M.B. Turner et al. / Journal of Volcanology and Geothermal Research 177 (2008) 1063–1076

exception that the dense clasts have larger and more numerousmicrolite phases (Fig. 3b). In exposures along the banks of theKaupokonui Stream (e.g., Fig. 1: 39°20′13″ South; 174°05′34″East) thebasal portion of this unit is a wavy-planar bedded, poorly sorted ashdeposit of a pyroclastic surge (Fig. 2).

CR unit 2 (Fig. 2) is a fall deposit that directly overlies unit 1 inmany locations on the eastern side of Mt Taranaki. The diffuseboundary and absence of medial ash, colluvium or soil developmentbetween the two units suggest that the CR unit 2 eruption occurredvery shortly (days to b1 yr) after the CR unit 1 eruption. It comprises ablack, basaltic-andesite scoriaceous lapilli layer with ~10% hypocrys-talline, non-juvenile, monolithic grey basaltic-andesite. Basaltic-andesite eruptions are rare in the Holocene record at Taranaki andwere primarily erupted from the satellite cone of Fanthams Peak(Neall et al., 1986). In thin-section the groundmass (55 vol.%) varies inits degree of crystallinity and comprises dull grey glass withmicrolitesof plagioclase, clinopyroxene, titanomagnetite±olivine and orthopyr-oxene (Fig. 3c). The crystal assemblage consists of approximately20 vol.% plagioclase (0.12–1.2 mm), ~10 vol.% clinopyroxene (0.2–1.7 mm), ~8 vol.% amphibole (~1.5 mm), a few vol.% olivine (~1.6 mm)as phenocrysts and a few vol.% titanomagnetite microphenocrysts(~0.3 mm). The hypocrystalline, non-juvenile component of theeruption is not discussed further.

The distribution (Fig. 1) and whole-rock chemistry for these unitsindicate that unit 1 wasmost likely erupted from a summit vent, whileunit 2 was sourced from a flank vent. Hence these two chemically andphysically different magmas were erupted almost contemporaneouslyfrom different vents during the Curtis Ridge eruption episode.Overlying these deposits at many locations along the KaupokonuiStream and Pembroke Rd sections (Fig. 1) is a poorly sorted, dune toplanar-bedded ash deposit (Fig. 2), characteristic of an ash-cloud surgeunit associated with block-and-ash-flows (Watanabe et al., 1999). Thelack of intervening fine-medial ash between the CR tephra andoverlying block-and-ash-flow deposits (Fig. 2) suggests that very littletime separates the events. Hence these block-and-ash-flow eventswere probably associated with the CR eruption episode. In this paperwe concentrate on the petrological characteristics of the openingphases (i.e., units 1 and 2) of this typical small-scale eruption episodefrom Mt Taranaki, New Zealand.

3. Petrology and geochemistry

3.1. Whole-rock geochemistry

Clean fragments of pumice, scoria and dense juvenile specimensfrom the two opening units of the CR eruptive episode were crushedbetween tungsten carbide plates and a 100 g split was ground to passthrough a b200 mesh in a tungsten carbide ring grinder. Major andtrace element concentrations were measured by X-ray fluorescence(Siemens SRS303AS spectrometer) using standard techniques on glass

Fig. 3.Optical micrographs of selected phenocrysts under cross-polarised light from CR units. White scale bar is 0.5 mm in a, b and c and 100 µm in d, e, f, g, h and i. (a) Overview of CRunit 1 pumice clast. (b) Overview of CR unit 1 dense grey juvenile clast. (c) Overview of CR unit 2 scoria clast. (d) Plagioclase with patchy-textured core in CR unit 1 pumice.(e) Plagioclase with patchy-textured core and rim in CR unit 1 pumice. (f) Plagioclase texture variability in CR unit 1 pumice. (g) Plagioclase phenocryst with patchy/sieve-texturedcore from CR unit 2 basaltic scoria. (h) Amphibole from CR unit 1 pumice. (i) Replaced amphibole from CR unit 1 dense grey juvenile lapilli.


fusion discs preparedwith SPECTRACHEM 12–22 (lithium tetraborate/lithium metaborate) flux following a method of Norrish and Hutton(1969). Trace elements were analysed using pressed powder pelletsaccording to methods based on Norrish and Chappell (1977). Majorelements are one-sigma relative errors b1%, whereas trace elementsare one-sigma relative errors of b1% for Sr and Zr, 1–3% for V, Cr, Cu,Zn, Ga and Y, 3–5% for Sc and Ni and 5–10% for Rb and Nb.Representative analyses of CR units and summit lava flows are given inTable 1.

Volcanic rocks erupted during the Holocene period at Mt Taranakiare high-K basaltic-andesites and andesites from the main summitcone and basalts from the parasitic cone of Fanthams Peak (Fig. 1).Over its ~130000 yrs of eruption history, Mt Taranaki eruptives haveprogressively become more potassic and siliceous (Zernack et al.,2006). The youngest rocks generally contain the highest potassiumand mean silica contents (Price et al., 1992; Stewart et al., 1996).However, within any given eruption period, variations across thewhole spectrum of compositions may occur, as is shown in theanalyses of this study. Major trace element and isotopic compositionsof the broad Taranaki rock suite are described in more detail by Priceet al. (1992), Stewart et al. (1996) and Price et al. (1999).

Major element variation of selected eruptives from the Holoceneperiod is shown using silica variation diagrams (Fig. 4). The clasts fromthe CR unit 1 event have almost homogenous major elementchemistry, with only minor variations in SiO2 (53.6–54.8 wt.%), CaO(8.3–8.8 wt.%), FeOtotal (8–8.6 wt.%) and Al2O3 (17.1–17.5 wt.%). Thedense grey juvenile clasts are at the highest end of the silica range. Thescoria clasts from CR unit 2 are also nearly homogenous in majorelement composition (Fig. 4), with much lower silica contents of 51–

51.5 wt.%. Trace element abundances are also uniform across thesample suite with minor variations for Sr (46–58 ppm) and Sr (598–628 ppm). FeOtotal, MgO, and CaO abundances decrease systematicallyand linearly with increasing SiO2, whereas Na2O and K2O abundancesincrease (Fig. 4). Al2O3 is much less variable.

Trace element abundances are characterised by relatively highproportions of large ion lithophile elements (LILE; e.g. Rb, Cs, Sr, Ba)and light rare earth elements (LREE), with deficiencies in high fieldstrength elements (HFSE; Ta, Nb, Zr; c.f. Price et al., 1992, 1999, 2005).These, as well as other trace element characteristics, such as depletionof Nb relative to La, and enrichment of Pb and Sr relative to Ce, aretypical arc-signatures (see Table 1).

The major and trace element concentrations of the CR unit 1samples are similar to those of most other deposits, erupted between1500 and 3000 yrs BP (Fig. 4). This observation is consistent with thatof Downey et al. (1994)z who, based on the lava flow sequence of MtTaranaki, first suggested that groups of geochemically similar magmaerupted during cycles with durations of 1000–2000 yrs (c.f., Priceet al., 1999).

3.2. Petrology

The mineral assemblages of thin-sectioned juvenile clasts fromCurtis Ridge unit 1 and 2 were characterised under optical microscopeand using the backscatter mode of a Scanning Electron Microscope(SEM). For mineral chemistry an additional 10 pumice and 10 scoriaclasts were crushed and sieved. Individual phenocrysts of hornblendeand plagioclase were hand-picked from the 500 µm–1 mm fraction.These crystals were embedded in epoxy resin and polished. Because of

Fig. 4. Major element variation as a function of SiO2 abundances for Taranaki eruptives. Samples are grouped by age.


the difficulty in extracting non-fractured crystals from the densejuvenile clasts of unit 1, phenocrysts of these samples were studied inclast thin-section. The composition of each mineral phenocryst was

Table 2Representative patchy/sieve-textured plagioclase phenocryst analyses

CR unit 1 pumice CR unit 2 scoria

Corea Coreb Rim Corea Coreb Rim Corea Coreb Rim

SiO2 56.47 52.86 55.90 54.61 48.55 55.98 58.20 47.57 44.49Al2O3 26.96 29.49 27.01 27.91 31.25 27.17 24.62 31.98 34.01FeOT 0.44 0.44 0.15 0.30 0.49 0.40 0.66 0.67 0.27MnO 0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.00 0.00MgO 0.01 0.00 0.05 0.12 0.07 0.00 0.00 0.01 0.00CaO 9.44 12.64 9.83 10.84 15.20 9.98 7.27 16.49 18.34Na2O 5.79 4.14 5.76 11.97 2.72 5.56 6.29 1.83 0.72K2O 0.63 0.24 0.56 0.40 0.19 0.51 1.55 0.35 0.20Total 99.69 99.82 99.27 99.15 98.47 99.63 98.67 98.90 98.13Si4+ 2.553 2.402 2.533 2.486 2.258 2.532 2.653 2.211 2.092Al3+ 1.433 1.579 1.442 1.498 1.713 1.449 1.322 1.752 1.885Fe2+ 0.017 0.017 0.006 0.011 0.019 0.015 0.025 0.026 0.015Mn2+ 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.00 0.00Mg2+ 0.001 0.000 0.004 0.008 0.005 0.000 0.000 0.001 0.00Ca2+ 0.457 0.615 0.477 0.529 0.758 0.484 0.355 0.821 0.924Na+ 0.508 0.365 0.506 0.439 0.245 0.488 0.556 0.165 0.066K+ 0.037 0.014 0.033 0.023 0.011 0.029 0.090 0.021 0.012Total 5.006 4.992 5.000 4.994 5.009 4.999 5.001 4.997 4.994An 0.46 0.62 0.47 0.53 0.75 0.48 0.35 0.82 0.92Ab 0.51 0.37 0.50 0.44 0.24 0.49 0.56 0.16 0.07Or 0.04 0.01 0.03 0.02 0.01 0.03 0.09 0.02 0.01

a Relatively sodic “blebs” of plagioclase.b Relatively calcic plagioclase.

determined by energy dispersive (EDS) electron microprobe (Jeol JXA-840A) at the University of Auckland. The analytical datawere collectedusing a Princeton GammaTech Prism 2000 Si (Li) EDS X-ray detector, a

Table 3Representative compositions and structural formulae of amphibole phenocrysts


Rim Mid Core Rim Core Rim Core Rim Core

SiO2 42.84 41.04 42.08 42.03 42.97 41.67 40.63 42.04 40.20TiO2 2.98 2.93 3.27 2.97 3.40 2.69 1.91 2.64 2.04Al2O3 10.25 13.00 10.97 10.72 10.82 13.36 15.00 13.68 14.38FeOT 11.98 13.05 12.86 11.67 13.04 11.87 10.60 11.49 10.70MnO 0.43 0.31 0.14 0.61 0.36 0.24 0.46 0.31 0.10MgO 15.04 12.51 12.99 13.87 12.25 14.06 14.40 13.38 13.39CaO 11.52 11.63 10.83 10.82 11.13 11.65 12.00 11.77 12.27Na2O 2.29 1.76 1.91 2.56 2.56 2.92 2.47 3.35 1.54K2O 0.99 0.96 1.17 1.10 1.19 1.02 0.92 0.97 1.24Total 98.32 97.19 96.22 96.32 97.72 99.48 98.38 99.63 95.86Si4+ 6.28 6.10 6.32 6.30 6.69 6.06 5.867 6.09 5.95Ti4+ 0.33 0.33 0.37 0.34 0.38 0.29 0.21 0.28 0.23AlIV 1.72 1.90 1.68 1.70 1.63 1.94 2.13 1.91 2.06AlVI 0.05 0.38 0.26 0.19 0.26 0.35 0.42 0.43 0.45Fe3+ 0.18 0.17 0 0 0 0.01 0.43 0.03 0.45Fe2+ 1.29 1.46 1.61 1.46 1.62 1.38 0.85 1.36 0.87Mn2+ 0.05 0.04 0.02 0.08 0.05 0.03 0.06 0.04 0.01Mg2+ 3.29 2.77 2.91 3.00 2.71 3.05 3.10 2.89 2.95Ca2+ 1.81 1.85 1.74 1.74 1.77 1.82 1.86 1.83 1.94NaM4 0 0 0.06 0.06 0.18 0.01 0 0.12 0NaA 0.64 0.51 0.49 0.68 0.56 0.81 0.69 0.82 0.44K+ 0.18 0.18 0.22 0.21 0.23 0.19 0.17 0.18 0.23Total 15.82 15.69 15.68 15.86 15.76 15.94 15.79 15.98 15.58Mg# 0.69 0.63 0.64 0.68 0.62 0.68 0.71 0.67 0.69

Table 4Representative clinopyroxene phenocryst analyses


Core Mid Rim Core Rim Core Rim Core Rim

SiO2 50.43 52.01 53.09 50.41 53.18 50.90 46.82 50.68 46.67TiO2 0.41 0.61 0.71 0.62 1.09 0.48 1.18 0.66 1.00Al2O3 3.09 2.28 1.55 3.15 1.36 2.96 7.18 2.67 6.62FeOT 7.33 7.42 6.83 7.83 6.71 7.09 8.18 7.85 8.16MnO 0.7 0.51 0.31 0.00 0.31 0.22 0.10 0.25 0.21MgO 13.98 14.29 15.39 13.05 15.27 14.71 12.44 15.40 12.40CaO 22.51 21.56 21.49 21.97 21.26 22.85 23.43 22.02 23.01Na2O 0.94 0.5 0.32 0.60 1.23 0.00 0.42 0.28 0.46K2O 0.24 0.23 0.17 0.25 0.00 0.15 0.08 0.00 0.00Total 99.64 99.40 99.86 97.88 100.43 99.36 99.84 99.80 98.48Si4+ 1.891 1.936 1.951 1.914 1.952 1.893 1.763 1.880 1.777Ti4+ 0.012 0.017 0.020 0.018 0.030 0.013 0.034 0.019 0.029Al3+ 0.136 0.100 0.067 0.141 0.059 0.130 0.319 0.117 0.297Fe2+ 0.23 0.231 0.210 0.249 0.206 0.221 0.258 0.244 0.260Mn2+ 0.022 0.016 0.010 0.00 0.010 0.007 0.003 0.008 0.007Mg2+ 0.782 0.793 0.843 0.738 0.836 0.816 0.698 0.852 0.705Ca2+ 0.904 0.860 0.846 0.894 0.836 0.911 0.946 0.876 0.940Na+ 0.069 0.036 0.023 0.044 0.088 0.000 0.031 0.020 0.034K+ 0.011 0.011 0.08 0.012 0.000 0.007 0.004 0.000 0.000Total 4.057 4.000 3.978 4.010 4.017 3.988 4.083 4.016 4.049En 0.38 0.39 0.42 0.37 0.41 0.41 0.34 0.42 0.35Mg# 0.77 0.78 0.80 0.75 0.80 0.79 0.73 0.78 0.73


2 µm focused beam, accelerating voltage of 12.5 kV, beam current of600 pA and 100 second live-count time. Calibration of the analysesused a suite of Astimex™ mineral standards. The following elementsare above detection limits, typical errors for rhyolitic compositions arein brackets SiO2 (b+0.13%), TiO2 (b+0.10%), Al2O3 (b+0.75%), FeO(b+2.5%), MnO (b+25%), MgO (b+10%), CaO (b+2.5%), Na2O (b+1.5%),K2O (b+2%), Cl (b+10%). This microprobe recently took part (alongwith 64 other laboratories) in the “G-Probe-2 international proficiencytest for microbeam laboratories”. The results were well within theacceptable deviation from the NKT-1G basaltic glass standard (c.f.,Potts et al., 2005) Representative analyses for each mineral are givenin Tables 2–5. EMP compositional line-scans, with an analysis every5 µm and 20 second live-count times, were carried out on phenocrystsfrom CR unit 1. The line-scan data were checked against spot analyses(Fig. 5a).

Table 5Representative titanomagnetite analyses


Euhedral Resorbed Euhedral

SiO2 0.07 0.01 0.04 0.08 0.05 0.05 0.08 0.06 0.19TiO2 6.22 6.17 6.21 6.98 7.01 7.01 6.12 6.25 6.35Al2O3 2.40 2.27 2.73 3.16 3.08 2.81 7.59 6.70 6.65FeOT 83.19 82.67 82.13 80.45 80.15 81.16 76.90 77.35 77.78MnO 0.81 0.90 0.9 0.72 0.72 0.77 0.29 0.52 0.45MgO 1.55 1.61 1.61 2.80 2.71 2.20 3.97 4.07 3.51CaO 0.07 0.16 0.16 0.00 0.02 0.00 0.04 0.10 0.02Cr2O3 0.07 0.15 0.22 0.01 0.10 0.07 – – –

Total 94.38 93.94 94.29 94.20 93.84 94.07 94.99 95.05 94.95Si4+ 0.021 0.003 0.012 0.024 0.015 0.015 0.023 0.017 0.054Ti4+ 1.399 1.386 1.391 1.552 1.567 1.572 1.304 1.331 1.365Al3+ 0.846 0.799 0.958 1.101 1.079 0.987 2.533 2.236 2.240Cr3+ 0.017 0.035 0.052 0.023 0.024 0.017 – – –

Fe3+ 12.298 12.514 12.184 11.725 11.735 11.822 10.953 10.953 10.778Fe2+ 8.501 8.139 8.269 8.160 8.179 8.415 7.550 7.366 7.809Mn2+ 0.205 0.228 0.237 0.180 0.181 0.195 0.070 0.125 0.109Mg2+ 0.691 0.717 0.870 1.234 1.200 0.978 1.676 1.718 1.495Ca2+ 0.022 0.051 0.019 0.00 0.006 0.000 0.012 0.30 0.006

3.2.1. PlagioclaseThe plagioclase phenocrysts in samples from the CR units show

complex compositional and textural variations. Two categories ofzoning were recognised:

1) Small-scale, cyclic oscillatory zoning (1–10 µm)with compositionalchanges of b10 mol% anorthite (An) and mostly ~8 mol% An.

2) Larger-scale, sieve-textured, partial resorption/dissolution zones(10–100 µm) with compositional changes of N20 mol% An. Thesesieve-textured zones contain abundant large, elliptical inclusions(N20 µm) of andesitic (~58% silica) melt and rare clinopyroxeneand magnetite. The number and thickness of the An-rich, sieve-textured zones also vary between individual crystals.

The unit 1 eruptives (dense juvenile and pumice) contain manylarger plagioclase phenocrysts (N1 mm) that exhibit both types oftextural zoning. These include patchy-textured cores, which mayinclude irregular blebs of melt and rare clinopyroxene andmagnetitemicrolites (Fig. 3d, e and f). Patchy-textured cores are eithercomprised of moderately (An70-60) or highly calcic plagioclase(An80-70) surrounded by channels or blebs of either melt, orrelatively sodic plagioclase (An65-55) (Table 2; Fig. 5). The cores aresurrounded by amantle, approximately 80 µm thick, of fine-scale (1–5 µm) oscillatory zoning (An55-45). A zone of partial resorption(sieve-textured) approximately 40 µm from the rim divides theoscillatory mantle in two (Fig. 3e). This sieve-textured zone isrelatively more calcic (An ~80-70) than the oscillatory zoning.Microphenocrysts and microlites of unit 1 eruptives have similarcompositions to those of the outer oscillatory zones of plagioclasephenocrysts (An54-48).

The cores of plagioclase phenocrysts of unit 2 dark-grey scoria aresimilar in appearance and composition to the cores of those in unit 1(Fig. 3g). Plagioclase phenocrysts rims and microphenocrysts displayfine oscillatory zoning. The rims are approximately 15 µm thick andare mostly free of glass and dissolution and/or partial resorptionzones. The rims are relatively calcic (An90-85) (Table 2).

3.2.2. AmphiboleA third of amphibole phenocrysts within the unit 1 pumice clasts

have cores that contain large melt, plagioclase and magnetite

Fig. 5. Compositional transects of selected plagioclase phenocrysts from the CR unit 1 pumice. Rim to core compositional profiles of two phenocrysts (a) and (b) were determined byEMP analyses. Black diamonds represent 20 second live-time EMP analyses every 2.5 µm along the white line. Grey squares represent the 100 second live-time EMP spot analyses ofthe locations shown as white spots on the photomicrograph. For (a) the compositions within the shaded area represent glass contamination.


inclusions (Fig. 3h) with Mg#1 of approximately 0.64. These coreshave diffuse, 50–100 µm zones that record Al2O3 changes of N1 wt.%,corresponding with an inverse change in SiO2 contents (Table 3;Fig. 6). These cores are surrounded by ~50 µm oscillatory zoned rimsthat are optically lighter than the cores. These rims have grown onirregular rounded, dissolution surfaces and are free of inclusions(Fig. 3h). Al2O3 decreases from 12 wt.% to approximately 10 wt.%. Mg#increases by approximately 5 wt.% across this boundary (Fig. 6). Theremaining amphibole phenocrysts and microphenocrysts (0.2–0.6 mm) have patchy, inclusion-rich cores, either surrounded byrims of oscillatory zoning, or displaying evidence of dissolution byrounded and embayed surfaces. Amphibole phenocrysts are mostlypseudomorphed, with some textural variability depending on thecrystal size. Smaller crystals (0.2–0.6 mm) have been completelyreplaced by microlites of titanomagnetite, clinopyroxene and plagio-clase (Fig. 3i). Larger phenocrysts (0.8–2.5 mm) have thick (~300 µm)resorption-textured rims of fine-grained titanomagnetite, pyroxene±plagioclase surrounding optically zoned oxy-hornblende cores.Amphibole phenocrysts do not display any textural zoning butcommonly contain inclusions of titanomagnetite and plagioclase.The cores of amphibole phenocrysts from scoria clasts of unit 2 haveAl2O3 and SiO2 of approximately 14.5 wt.% and 40.5 wt.%, respectivelyand Mg# of 0.70 (Table 3). There is a gradual decrease of Al2O3

associated with an increase of SiO2 from core to rim of 1–1.5 wt.%.

1 Mg#=Mg/(Mg+Fe) normalized cations per formula unit; minimum Fe3+ estimatedafter Schumacher (1997).

3.2.3. ClinopyroxeneClinopyroxene phenocrysts of unit 1 eruptives commonly form

glomerocrysts with titanomagnetite and rare plagioclase crystals. Thecores of these phenocrysts have Mg# of approximately 0.75–0.77 andare either homogenous, or show diffuse zones with changes in Al2O3

of ~1 wt.% (Fig. 7). These cores are surrounded by 50–200 µm thickoscillatory zoned rims, which are associated with increases of Mg# to0.80, accompanied by a N1 wt.% decrease in Al2O3 (Table 4).

Clinopyroxene phenocrysts of unit 2 are often found in associationwith titanomagnetite±plagioclase phenocrysts as glomerocrysts(~4 mm). These phenocrysts have patchy-textured cores with similarMg# (i.e. 0.79; Table 4) to those of the CR unit 1 eruptives. Broadcompositional zones where large changes of Al2O3 from 3.5 to N6 wt.%(Table 4) are also present.

3.2.4. TitanomagnetiteTitanomagnetite, the most common Fe–Ti oxide, is micropheno-

crystic (b1.0 mm) and commonly forms glomerocrysts with clinopyr-oxene±plagioclase. The titanomagnetite of CR unit 1 pumices can bedivided into two distinct populations based on TiO2 content andmorphology. 1) Resorbed, high-TiO2 grains with TiO2 contents from6.4 to 6.8 wt.% (1.45–1.55 Ti pfu), which are also rich in apatiteinclusions (Table 5; Fig. 8). 2) Euhedral, low-TiO2 grains with 5.9 to6.2 wt.% (1.25–1.40 pfu) TiO2 (Table 5; Fig. 8). Both types are commonwithin the microphenocrysts assemblage; however, resorbed/sub-euhedral grains are common within the cores of clinopyroxene andamphibole phenocrysts, whereas euhedral grains are typicallyincorporated within the clinopyroxene rims and glomerocrysts withclinopyroxene and plagioclase. Titanomagnetite from the basaltic-

Fig. 6. Compositional transect of an amphibole phenocryst from CR unit 1 pumice. EMPanalyses with 20 second live-count times were collected every 4 µm along the whiteline. Grey squares indicate SiO2 contents; black crosses are Al2O3 and Mg# are blackdiamonds. The grey dashed line on the photomicrograph highlights gradual diffusezoning. The rim is apparent by a thick black zone in the photomicrograph.


andesite scoria of CR unit 2 has a distinctly higher Al2O3 content ofbetween 7 and 7.7 wt.% (2.2–2.6 pfu), while the TiO2 content is moreuniform at 6–6.4 wt.% (1.25–1.35 pfu).

4. Discussion

4.1. Petrology

In recent years, models of volcanic magma systems have beenrefined by the use of fine-scale petrological observations of mineraland rock textures, and intra-grain chemical variations (Tepley et al.,2000; Devine et al., 2003; Zellmer et al., 2003; Landi et al., 2004; Saitoet al., 2005). Such studies use petrological observations to infer the

conditions (temperature, pressure and melt compositions) thatprevailed in magmas before and during their ascent to eruption.Here we use the petrological variations observed within the mineralsand rocks from the studied CR units in conjunctionwith field evidenceto interpret the magmatic conditions that led to the initiation of aneruption episode. We have therefore concentrated on the primarypumice clasts of the CR unit 1 deposits for much of this discussion.Fig. 9 summarises the discussion of phenocryst chemical and texturalvariations.

4.1.1. Plagioclase zoningThe compositional and textual complexity of plagioclase have been

well documented inmany andesite systems (Tepley et al., 2000; Stewartand Fowler, 2001; Zellmer et al., 2003; Landi et al., 2004). The very slowdiffusion rate of major elements (e.g., Ca–Al exchange is 10−20 cm/s;Morse, 1984) within the plagioclase lattice means that compositionalzoning preserves the magma conditions that prevailed during thegrowthof the crystal. Furthermore, thewide occurrenceof plagioclase involcanic rocks and its potential to crystallise over the entire magmatichistory of an eruptive episode, has resulted in it becoming an importanttool in determining conditions ofmagmatic storage, ascent and eruptiveprocesses (Singer et al., 1995; Tepley et al., 2000; Stewart and Fowler,2001; Couch et al., 2003; Landi et al., 2004). The compositional andtextural zoning of plagioclase phenocrysts observed within Taranakieruptives are similar to those described elsewhere (e.g., Stewart andFowler, 2001; Landi et al., 2004) and include oscillatory zoningwith bothmajor and minor resorption surfaces, as well as patchy or partialresorption/dissolution zones and cores. These features appear in alleruptives from the CR episode.

Small-scale, high frequency, cyclic oscillatory zoning (zones 1–10 µm with compositional oscillations of b3–5 mol% An) occursthroughout the growth of the examined plagioclase phenocrysts (e.g.,Fig. 3e) and is thought to reflect near equilibrium, diffusion controlledgrowth (Pearce and Kolisnik, 1990, Pearce, 1993). Kinetic effects occurat the crystal–melt boundary layer, where the liquid compositionimmediately surrounding the crystal is different from that of the bulkmelt (Stewart and Fowler, 2001). These effects are difficult to constrainand have been theoretically, numerically and experimentally mod-elled in terms of plagioclase equilibria and growth kinetics (e.g.,Housh and Luhr, 1991; L'Heureux and Fowler, 1994). The highfrequency oscillations and repeated minor resorption surfaces canalso be attributed to thermal and compositional changes in the orderof ≤5 mol% An and/or 5–25 °C (Ginibre et al., 2002). In a super-saturated liquid, a local boundary layer is formed at the crystal–meltinterface which is periodically destroyed by a shear pulse that bringsnew melt to this interface. A combination of this mechanism togetherwith the small thermal change associated with the replenishment ofnewmelt to the crystal surface, may also explain the oscillatory zoningand minor resorption surfaces (Ginibre et al., 2002). Recently, Blundyet al. (2006) suggested that oscillatory zoning in plagioclasephenocrysts could be related to the opposing effects of decompressionand heating on the equilibrium liquidus of plagioclase. Decompressionof a hydrous magma decreases An in the crystallising plagioclase,while the latent heat released by crystallisation increases An. There-fore, the characteristic normal oscillatory zoning is the result of acomplex interplay between the two effects (Blundy et al., 2006). Smallmelt inclusions may be trapped between the overgrowth andresorption surface resulting in skeletal textures and fine sieve textures(Stewart and Fowler, 2001).

Larger-scale, partial resorption zones (zones 10–100 µm andcompositional changes of 10–20 mol% An) are commonly referred toas having a sieve texture, due to the presence of abundant meltinclusions (Tepley et al., 2000; Stewart and Fowler, 2001; Zellmeret al., 2003; Landi et al., 2004). These zones are usually associated withwell developed dissolution surfaces. Compared to the small-scaleoscillatory zoning formed in close-to-equilibrium conditions, abrupt

Fig. 7. Compositional transects through selected clinopyroxene phenocrysts from (a) CR unit 1 pumice and (b) CR unit 2 scoria. 20 second live-count time EMP analyses every 2 µm(a) and 4 µm (b). Mg# is represented as black diamonds and Al2O3 contents as black crosses.


compositional and textural changes in plagioclase phenocrysts arelikely to be due to dramatic changes to the surrounding melt resultingfrom changes in temperature, pressure, volatile contents or meltcomposition (Pearce and Kolisnik, 1990; Singer et al., 1995). Dissolu-tion zones are thought to be produced by relatively Na-richplagioclase being subjected to a higher-temperature melt, where Ca-rich plagioclase should be in equilibrium (Tsuchiyama, 1985). Coarse,relatively calcic sieve-textured zones, with large elliptical meltinclusions (N20 µm), have been experimentally shown to require thesurrounding melt to either have increased in temperature or in PH2O(e.g. increase of volatile content) compared to the original conditionsunder which the oscillatory zones crystallised (Tepley et al., 2000;Stewart and Fowler, 2001; Zellmer et al., 2003). A large increase involatile content would result in a lowering of the plagioclase solidustemperature, favouring Ca-rich plagioclase (Tepley et al., 2000).Subsequent release of the volatiles during eruption would result inthe crystallisation of a lower An-plagioclase. Reheating the plagioclaseabove its equilibrium solidus temperature, but below its liquidus, willgenerate rapid re-crystallisation, producing a sieve-texture zone ofvery fine-grained, relatively An-rich plagioclase and trapped glass.Although these compositional and textural characteristics andcompositional changes of plagioclases can be produced by closed-system processes (c.f., Tepley et al., 2000), with independent evidencewithin other phenocryst phases or isotopic variations, they arecommonly ascribed to the intrusion and/or ponding of a hottermagma at the base of a resident magma (Stewart and Fowler, 2001;Couch et al., 2003; Zellmer et al., 2003; Landi et al., 2004).

The majority of plagioclase phenocrysts studied rocks (includingthe scoriaceous CR unit 2 clasts) have patchy-textured cores (Figs. 3d,e, f and 5). Hence we suggest that these cores represent plagioclasesthat have grown at depth and were partially resorbed during ascent.Humphreys et al. (2006) describe similar textures within plagioclasephenocryst from Shiveluch Volcano. They suggest that patchy cores

are the result of partial resorption from ascent of an H2O rich butundersaturated magma. The resulting plagioclase contains a patchyAn-rich area, along with relatively Ab-rich plagioclase and trappedmelt inclusions (Price et al., 2005; Humphreys et al., 2006). This is verysimilar to the cores of the studied Taranaki plagioclase phenocrysts.

4.1.2. AmphiboleAmphibole phenocrysts of the dense juvenile lapilli from unit 1

have thick rims (~100–200 µm), or have been completely replaced bymagnetite, pyroxene and plagioclase. These rims occur when theamphibole phenocrysts are subjected to pressures outside of theirstability field (Rutherford and Devine,1988; Rutherford and Hill, 1993;Browne and Gardner, 2006). The grain-sizes of the replacementminerals and thickness of the rims are directly related to the depthand the amount of time a magma is held at this depth before beingerupted (Browne and Gardner, 2006). Comparing the observationsfrom Taranaki to those of Redoubt dacite (Browne and Gardner, 2006)and Mount St. Helens dacite (Rutherford and Hill, 1993), we infer thatamphibole phenocrysts of the CR unit 1 dense juvenile componentstalled at higher levels with pressures outside that of amphibolestability (b100 Mpa for Taranaki amphibole; T. Platz; pers. comm.,2007) for a period greater than 7 days before being quenched. Thisimplies that the grey dense juvenile clasts represent the upperdegassed portion of the magma system, similar to the conclusionsfrom recent studies by Platz et al. (2007). By contrast, amphibole of theCR unit 1 pumice clasts have no decompression rims and thereforemust have risen quickly from depths where it was stable (3–7 km;Platz et al., 2007).

Compositional profiles of amphibole phenocrysts can be used toinfer pressure, oxygen fugacity, pH2O and temperature conditions ofthe magma system during phenocryst growth (e.g., Foden and Green,1992; Holland and Blundy, 1994). Al2O3 variations of the patchy-textured, diffusely zoned cores of Taranaki amphiboles (Fig. 6) can be

Fig. 8. Al and Ti contents of titanomagnetite microphenocrysts from CR units. Filledcircles: Analyses from CR unit 2 scoria. Crosses: Analyses from CR unit 1 pumice.Compositions and Scanning Electron Microscope (SEM) backscatter photomicrographsof euhedral and resorbed grains are highlighted (discussed in text). Al and Ti pfu:Normalised Al and Ti contents of titanomagnetites per unit formula (atoms/24 cations,36 oxygen formula units).


explained by fluctuations in temperature, because the compositionalexchange vector SiIV+( )A=AlIV+(Na,K)A is strongly controlled by thisfactor. Increasing temperature results in higher Al2O3 in amphibole(Blundy and Holland, 1990). Holland and Blundy (1994) provide analternative explanation where the composition of amphibole reflectsthe Al2O3 content of the magma, which, in-turn, is related to thecrystallisation of plagioclase by the following compositional exchangereaction:

NaCa2ðAlSi3ÞSi4O22ðOHÞ2ðedeniteþ NaAlSi3O8ðalbiteÞ¼ NaðCaNaÞMg5Si8O22ðOHÞ2ðrichteriteÞ þ CaAl2Si2O8ðanorthiteÞ:

Decreases in Al2O3 contents of amphibole could, therefore, relateto an increase in the growth of a relatively An-rich plagioclase.

A rounded interface separates the cores and the brighter colouredrims of most CR unit 1 amphiboles (Fig. 6). This is also associated witha decrease in Al2O3 content and increase in Mg#. The effect oftemperature on the Mg# of amphibole is likely to be small(Humphreys et al., 2006), whereas increases in fO2 produce strongincreases in Mg# (Scaillet and Evans, 1999). Therefore, the rims areinferred to have crystallised under conditions of relatively high fO2,compared to those during which the amphibole cores were crystal-

lised. The amphibole rims contain lower Al2O3 than the cores. This ismost likely the result of amphibole crystallisation under conditionswhich are both lower in temperature and Al2O3, with the decrease ofAl2O3 relating to substantial An-rich plagioclase crystallisation.

4.1.3. Clinopyroxene compositional profilesIn general, Taranaki clinopyroxene phenocrysts have diffusively

zoned cores. The zones are between 50 and 200 µm wide and recordsubtle changes in compositions, particularly that of Al2O3 and Mg#(Fig. 7). Changes of Mg#within clinopryoxene reflect temperature andfO2 fluctuations during crystallisation, with increases of Mg# resultingfrom increases in temperature and fO2 (Nakagawa et al., 2002). Al2O3

is assumed to be proportional to the Al2O3 content of the melt (Cortéset al., 2005) and Al2O3 availability is related to plagioclase crystal-lisation within the surrounding melt (c.f., Humphreys et al., 2006).Therefore, decreases in clinopryoxene Al2O3 content relate to anincrease in the crystallisation of An-rich plagioclase. This may resultfrom an increase in magmatic temperature (Tepley et al., 2000;Stewart and Fowler, 2001; Zellmer et al., 2003), or during thedecompression ascent of an H2O-undersaturated magma (Humphreyset al., 2006).

The diffusively zoned clinopyroxene cores are surrounded bydistinct 50–200 µm oscillatory zoned rims. Partial resorption surfaceswith abundant melt inclusions separate the core from the rims. Therims have higherMg#, suggesting that they crystallised from a slightlyhottermagma than their cores (Fig. 7). Al2O3 also decreasedwithin theclinopyroxene rims, which is expected from the An-rich plagioclasecrystallisation that has depleted the melt in Al2O3 also resulting froman increase in temperature (c.f., Tepley et al., 2000).

4.1.4. TitanomagnetiteTwo distinct populations of CR unit 1 titanomagnetites are defined

by their TiO2 content and morphology (Fig. 8). 1) Resorbedtitanomagnetites have relatively high-TiO2 content and are rich inapatite inclusions while slightly lower-TiO2 titanomagnetites 2) areeuhedral and relatively free of inclusions. Resorbed titanomagnetitesare a common accessory phase within the cores of clinopyroxenephenocrysts, whereas euhedral types are present in clinopyroxenerims. Both phases are also prolifically present as microphenocrysts.

Oxygen fugacity and temperature changes influence the composi-tion of titanomagnetites in hydrous andesitic systems (Frost andLindsley, 1991). Increasing temperature at constant fO2 or decreasingfO2 at constant temperature results in an increase in titanomagnetiteTiO2 contents (Devine et al., 2003). The different TiO2 contents of thetitanomagnetite microphenocrysts therefore relate to either changesin oxygen fugacity or temperature, or both. Mg# compositionalprofiles of clinopyroxene phenocrysts indicate that the cores of thesephenocrysts were formed under relatively lower temperatures andlower fO2 than that of their rims. The relatively high-TiO2 content ofthe resorbed titanomagnetites that co-crystallised with these coresmust therefore reflect the relatively low fO2 of the melt, rather thancrystallising under relatively higher temperatures. This interpretationis supported by amphibole data, where relatively lower Mg# of thephenocryst cores indicate that relatively low fO2 existed during theircrystallisation. In contrast the euhedral co-crystallising titanomagne-tites of the clinopyroxenes are low in TiO2 (Fig. 8). We argue that this isa result of rapidly increasing fO2 that more strongly influencedtitanomagnetite compositions than the increasing temperature.Increasing Mg# in the amphibole rims also implies an increasing fO2

environment during phenocryst rim formation.

4.2. Magma recharging and the generation of an eruption episode

Models of volcanic magmatic systems have conventionallyfocussed on relatively shallow-level magma chambers that fractionateduring slow cooling (e.g., Shaw, 1985). This model can explain many of

Fig. 9. Summary of phenocryst textures, their chemical characteristics and interpreted origins.


the observations at rhyolite volcanoes (e.g., Smith et al., 2004), or forshort magmatic residence times (e.g., Zellmer et al., 2003), and short-term eruptive variability (Hobden et al., 1999; Platz et al., 2007).Mingling andmixing textures common in andesites, however, indicatemore complex magma systems (e.g., Price et al., 2005). Yet sinceandesitic stratovolcanoes erupt similar crystal-rich andesite overthousands of years, it suggests that their magma system is periodicallyrecharged with new material from depth (Eichelberger et al., 2006).This may be basaltic magma derived from partial melting of themantle wedge, in the case of high-Mg-basalts (Grove, 2002), ormodified basalt or andesite derived from a combination of crystal-lisation of the primary basalt (Grove et al., 2002) and melting crustalrocks within the lower crust or upper mantle (Price et al., 2005).

The petrological evidence from the opening phase of the CurtisRidge eruption episode (CR unit 1) provides strong evidence for themixing of two magmas, shortly before eruption, that differed onlyslightly in both temperature and fO2. Many plagioclase phenocrystshave a partial resorption zone within a few µm from the edge of thephenocryst. Amphibole and clinopyroxene phenocrysts have dissolu-tion zones surrounded by compositionally different rims. Plagioclase,

clinopyroxene and amphibole resorption are the result of a relativeincrease in magmatic temperature and fO2. Two compositionallydifferent titanomagnetite microphenocrysts also suggest an interac-tion of two magmas.

The traditional model of magma mixing involves a residingandesite magma being reheated by injection and/or ponding of abasaltic magma (Eichelberger, 1978; Murphy et al., 2000). However,this model does not adequately account for the patchy and/or diffuselyzoned cores of the plagioclase, amphibole and clinopyroxenephenocrysts, or the occurrence of two compositionally and morpho-logically distinct sets of titanomagnetite microphenocrysts. In addi-tion, the eruption of basaltic material immediately after,or simultaneously with an andesitic eruption is globally rare(Eichelberger et al., 2006). This is because once magma fractures thecountry rock to rise to the surface, subsequent magma flow will bepreferably channelized through the fracture rather than elsewhere(Eichelberger et al., 2006). Therefore if the CR unit 1 andesite magmawas injected by the basalt of the CR unit 2, it would seem likely thatthis basaltic magma should have erupted from the same vent as theandesite. Injection of the CR unit 2 magma into the andesitic magma

Fig. 10. Schematic diagram of andesite magma recharge. The recharging magmacontains phenocrysts of amphibole, plagioclase, clinopyroxene and titanomagnetitethat are resorbed due to decompression upon ascent (a). Rims of phenocrysts arecrystallised under the magmatic environment of the upper storage area (b). Areas of theresiding magma which are not affected by the recharging event are shown in (c).


should also have led to the entrainment of basaltic-andesite blebs toform enclaves (e.g., Nakagawa et al., 2002; Eichelberger et al., 2006).These were not observed. We therefore propose the followingmagmatic process, which is based on similar mechanisms to thosedescribed by Price et al. (2005) and Humphreys et al. (2006), involvingthe recharge of the residing andesite magma by an andesite of similarcomposition.

4.2.1. Recharge of andesite magmaHigh level crustal magma storage and the presence of dispersed,

complex plumbing systems were thought to cause the observedgeochemical and phenocryst textural variations within andesitic rocks(Price et al., 2005). However, many of these features can be accountedfor by sub-volcanic magma systems that grow incrementally via thecoalescence of many small magma bodies (Coleman et al., 2004;Humphreys et al., 2006). The model adapted here is consistent withPrice et al. (2005) who proposed that andesite magmas are generatedin the lower crust. These migrate upwards as crystal- and lithic-richmagmas, containing melt phases of up to rhyolite and dacitecomposition. If a recharging magma has similar properties to thoseof the residing magma body, then evidence of mixing will be difficultto determine due to the limited disequilibrium between crystalsand melt (D'Lemos, 1996). Cryptic magma mixing hence results(Humphreys et al., 2006). The efficiency of mixing under suchcircumstances depends on the volume ratio of intruding to residentmagma, as well as relative differences in temperature, dissolvedvolatile content and composition between the two magmas (Sparksand Marshall, 1986). We propose that the upper andesite magmasystem (5–7 km depth) of Taranaki was recharged by a similar, butslightly hotter and more volatile-rich magma. The geochemical andphysical differences must have been small enough to allow efficientcryptic mixing. The textural characteristics of the patchy plagioclasecores indicate resorption resulting from H2O-undersaturated decom-pression (c.f., Humphreys et al., 2006). The patchy cores of theamphibole and clinopryoxene phenocrysts are also explained by asimilar decompression resorption scenario (Candela, 1986). Theapatite inclusions and resorption of the high-Ti titanomagnetitesuggest that they were crystallised from a different magma to thatof the lower-Ti, euhedral grains. We therefore suggest that these fourphases were originally crystallised at depth (Fig. 10a). Upon ascenteach phase was resorbed (Fig. 10b) due to decompression-drivensuperheating, relative to the liquidus (Annen et al., 2006), then byvolatile dissolution and subsequent decompression crystallisation.

The phenocryst assemblage of the residing andesitic magmaconsisted of plagioclase, clinopyroxene and amphibole phenocrysts,which were either from a prior magma recharge event (containingphenocryst cores that display patchy/resorption textures of decom-pression), or they crystallised under near-constant magmatic condi-tions within the residing andesitic magma (i.e., oscillatory zonedplagioclase phenocrysts). The residing magma also had a higher fO2,compared to the recharging body. Euhedral titanomagnetite grains arecrystallised within the upper residing magma system as bothmicrophenocrysts, inclusions within the rims of clinopyroxene, orwithin glomerocrysts with plagioclase and clinopyroxene. Recharge ofan andesitic magma that is relatively hotter and has elevated PH2O willresult in the formation of a sieve-textured zone near the rims of theplagioclase phenocrysts (Tepley et al., 2000; Stewart and Fowler,2001; Zellmer et al., 2003). The high-Mg# and low Al contents ofamphibole and clinopyroxene rims of the intruding phenocrysts are inresponse to an increase in both temperature and fO2 of the newmagmatic environment and co-crystallising plagioclase. Fig. 10illustrates the possible spatial relationship between the phenocrystassemblages for this recharge model.

Within the whole-rock geochemical sample suite, there aredominant, linear trends of major element variation (Fig. 4). Theseare consistent with a broad control by crystal fractionation processes

(Bowen, 1928; Gill, 1981; Gamble et al., 1990). However, Price et al.(1999) suggest that because of the complexity of crystal fractionation,later crystal resorption and assimilation processes (AFC processes;DePaolo, 1981), modelling the geochemical variations in the Taranakieruptives are extremely problematic. This is because much of thephenocryst assemblage is apparently unrelated to the surroundingmelt, making whole-rock analyses not the result of simple liquid linesof descent (see example in Price et al., 1999). The results presentedhere imply that, although phenocryst rims are in equilibriumwith thesurrounding erupted melt, many of the grains have crystallised underdifferent conditions at lower-crustal levels. Under the proposedmodel, much of the magma differentiation occurs at lower levelswithin the crust fromwhich melts and crystal cargo subsequently riseand recharge the upper system. Differences in AFC processes withinthe deeper part of the system and subsequent rise are responsible formuch of this variation. Of course, later, shallow crustal processeswould also effect whole-rock geochemical and petrographical varia-tions. These processes may include further fractional crystallisation(Stewart et al., 1996) and magma mingling.

4.2.2. Timescales of recharge and/or heating before eruption: trigger tothe eruption?

The high diffusion coefficients of Ti in titanomagnetite mean thattheir compositions reflect themagma conditions just prior to eruption(Tomiya and Takahashi, 2005). The Ti diffusion coefficient at Taranakiindicates temperatures of between 800 and 900 °C (c.f., Stewart et al.,1996) is between 1.3 and 10.3×10−17 m2/s (Freer and Hauptman,1978). Homogenization of Ti within a spherical solid with a radiusequivalent to Taranaki titanomagnetite phenocrysts (c. 100 µm) wouldbe between 1.5 and 12 yrs (e.g., Crank, 1975, p. 92). This can be used toconstrain the timescales of recharge within the bimodal population oftitanomagnetites. The high-Ti titanomagnetite grains are resorbed andcontain apatite inclusions, whereas low-Ti titanomagnetite grains areeuhedral and inclusion free. The occurrence of these two titanomag-netite types within both cores and rims of clinopryoxene andamphibole phenocrysts also provides strong evidence that the


compositional differences result from differences in fO2 during theircrystallisation. Therefore, we interpret that sub-euhedral titanomag-netites were crystallised at depth and subsequently ascended andcryptically mixed with the phenocryst assemblage of the CR unit 1pumice (Fig. 10b). Rapid Ti diffusion means that changes of thesurrounding magmatic temperature or fO2 would result in strong rimto core Ti diffusion gradients (c.f., Devine et al., 2003). The lack of thesestrongly suggests that the two magmas were mixed together onlybriefly (i.e. hours/weeks) before eruption/quenching.

Magma recharge events are widely acknowledged as potentialeruption triggers (Sparks, 1977; Eichelberger and Izbekov, 2000;Murphy et al., 2000). The addition of new material to the base of amagma system increases its pressurisation. Reheating residual magmacauses volatile exsolution and, along with addition of volatiles fromthe recharging magma, further increases pressurisation and hencepotential for fracturing of country rock and eruption (Murphy et al.,2000).

5. Conclusions

We have used detailed field observations and fine-scale petrolo-gical studies of the c. 1800 yrs BP Curtis Ridge eruption episode as thebasis for a model of the upper magma system of Taranaki. This episodewas initiated with an andesitic sub-plinian eruption from the summitvent, followed by a basaltic-andesite eruption from the volcano'sflank. The remainder of the episode continued with a period of domebuilding and associated block-and-ash-flow generation.

The phenocryst assemblage of the deposits from the openingphase of the CR eruption episode provides clear petrological evidenceof magma mixing just prior to its onset. These petrological observa-tions include calcic, sieve-textured zones near the rims of plagioclasephenocrysts and rimswith high-Mg# in clinopyroxene and amphibolephenocrysts. These all suggest that rim crystallisation occurred undera hotter, high fO2 magmatic environment than during core formation.In addition, a bimodal titanomagnetite population implies twodifferent magmatic components.

We propose that two compositionally and physically similarandesitic magmas were cryptically mixed within the upper magmaticsystem of Mt Taranaki prior to the CR episode. Andesitic magma wasdifferentiated at depth and subsequently rose to recharge the upper7–10 kmmagmatic system. Decompression-induced resorption of therecharging magma formed patchy-textured plagioclase phenocrystscores and rounding of the amphibole and clinopyroxene phenocrysts.Subsequent crystallisation within the upper magma storage areaunder higher oxygen fugacity and temperature generated rims ofdiffering compositions. Plagioclase phenocrysts within the residingmagma also indicate changes within the temperature relating to themixing event.

Two discrete groups of titanomagnetite microphenocrysts co-existwithin the erupted CR unit 1 juvenile lithics; a resorbed, high-Ti groupwhich contains abundant apatite inclusions and a euhedral, low-Tigroup. A lack of Ti diffusion in these co-existing minerals impliesrecharging and/or reheating occurred within days to weeks before theeruption and hence the recharge event was also likely the eruptiontrigger. Similar evidence of magma recharge is also seen in thephenocrysts of other major eruptions from this volcano, suggestingthis process of magma recharge is common and responsible for theinitiation many of the eruption episodes from Mt Taranaki and othervolcanoes of a similar nature.

Acknowledgements

We thank Dr. Ritchie Sims of the Geology Department of AucklandUniversity, NZ for his enthusiastic help with the Electron Microprobe.We also thank Dr. Thomas Platz of Massey University, NZ for hishelpful discussions. This work is prepared as part of a PhD thesis at

Massey University and is partly funded by NZ FRST contractMAUX0401 (to SJC) and, a Massey Doctoral Scholarship and theGeorge Mason Trust, Taranaki. We also thank reviewers F Tepley, G.Wörner, S. Chakraborty and John Gamble for their helpful andconstructive comments on an earlier version of this manuscript.

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eruption episodes and magma recharge events in andesitic systems: mt taranaki, new zealand

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